Current-regulating means for electric induction furnaces



my 21, aym m C. B. FOLEY CURRENT REGULATING MEANS FOR ELECTRIC INDUCTIONFURNACES Oriainal Filed March 25, 1922 lllllll Uimrias B ui Ze 17,

- M (a 5M Patented Feb. 21, 1928.

, UNITED STATES PATENT OFFICE.-

CHARLES B. FOLEY, OF FORT WAYNE, INDIANA, ASSIGNOR TO CHARLES B. FOLEY,INO., OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

OURRENT-REGULATING MEANS FOR ELECTRIC INDUCTION FURNACES.

Application filed March 23, 1922, Serial No. 545,985. Renewed J'u'ly 14,1926.

This invention relates to electric furnaces and particularly to thosefurnaces of the I induction type in which a loop or channel of molten.metal forms the secondary of the transformer which supplies energy tothe furnace.

One object of this invention is to provide a simple current regulatingmeans for a furnace of this type. Another object is to increase theoperatin efliciency of the furnace. Still another 0 ject is to increasethe power factor of the furnace and still another object is to providemeans for easil adjustingl the furnace to melt metals of di ferent caracteristic's, for example, metals having different electricalresistance.

These objects are accomplished by providing an additional coil for thetransformer which is so disposed that the leakage flux is 10 verymaterially reduced and by roviding means for varying the number 0effective turns of this coil.

In' electric furnaces of the type contemplated by this invention, theprimary coil 55 ofthe t ansformer is placedwvithin the bore of a reractory tube which, together with the furnace walls, forms the channelfor the molten transformer secondary. A laminated iron core is passedthrough the primary coil 80 and is closed upon itself outside of therefractory tube to form a closed magnetic circuit for the transformer.

In any transformer the current in the primary coil'produces magneticflux in the 85 iron core in one direction and the current in thesecondary coil produces magnetic flux which is opposed to that producedby'the primary and this condition always produces a certain amount ofstray or leakage flux,

40 that is, flux which does not stay within the iron core but completesits magnetic circuit without threading or interlinking both the primaryand secondary coils. This leakage flux generally finds a path throughthe space between the primary and secondary coils.

In the ordinary ower transmitting transformer it ispossible to constructthe iron core and the primary and secondary coils in such manner thatthe space between these coils is very small and the leakage fluxconsequently negligible. However, in the construction of the electricfurnace, while it is possible to place the primary coil very close turnsto the coil that produced it without I threading both the primary andsecondary.

Placing the primary coil within the refractory tube, as described above,locates it as close to the secondary as possible but even with thisconstruction the leakage flux is very high, sometimes reaching as muchas 50% of -the flux produced by the primary coil. This loss of fluxcauses a serious drop in the voltage generated in the secondary coil andreduces the secondary current Which in turn reduces the amount of heatimparted to the metal.

It is well known that two primary coils when placed on the same core andconnected in multiple will, with the secondary open circuited, eachproduce its proportional share of the required total magnetic. flux. Ihave discovered that by placing one of these coils .on the outside legof the transformer core of a furnace of the above mentioned type suchcoil not only still tends to supply 1ts share of flux but at the sametime tends to greatly reduce the amount of stray or leakage fiux; inother words, the flux produced by the primary coil, instead of findingits way back through the space between the primary and secondary coils,remains in the iron core and threads both of these coils.

This increase in the total flux threading both rimary and secondary, aswould be expecte causes the secondary voltage to approach the normalcalculated value, increases the heating current in the molten secondaryand improves the power factor of the furnace.

By providing suitable switching contacts the number of turns of thiscoil may be readily varied and a convenient control of the furnacecurrent secured thereby.

It very frequently happens that at one time an electric furnace will becalled upon to melt a metal of very low electrical resistance such asbrass, while at another time it may be desired to melt a metal of verymuch higher electrical resistance such as German silver. Also where afurnace has been in continuous operation for several weeks, the wear onthewalls of the channel increases the cross section of the seconda andcauses the primary to become overloa ed. A control to take care of suchconditions as these must possess a considerable range, and in order toincrease the range of control, the coil which is provided on the outsideleg. of the transformer may be made use of as an auto-transformer inwhich the secondary may be connected by asuitable switch mechanism toeither buck or boost the voltage applied to the main primary coil of thetrans former located within the refractory tube.

In the accompanying drawings- Fig.1 is a view in central verticalsection of an induction furnace of the crucible type such as isdescribed in my copending application, Serial No. 545,984, filed March23, 1922. It will be understood, however, that the invention is equallyapplicable to induction furnaces of other types.

Fig. 2 is a diagrammatic view illustrating the current regulating means.employed in accordance with this invention; and

Fig. 3 is a diagrammatic view of a modification of the circuitarrangement shown in Fig. 2.

The furnace shown in Fig. 1 consists of an upper section 1 cemented to alower section 2, the two sections being firmly held together by tie-rods3. Numeral 4 designates the tapping nozzle of the furnace and 5, thevalve through which the flow of metal from the,

nozzle iscontrolled. The valve is operated a ainst the tension of a coilspring by means of the lever 6. The furnace as fully described in mycopending application above identified has fixed in its lower sectionarefractory tube 7 located in registry with coaxial windows 8 cutthrough the sides of said lower section, the tube forming in conjunctionwith the bottom and sides of the lower section a plurality ofloop-shaped channels 9 whereby the metal contained: in the furnace isbrought into inductive relation with current inducing means constitutedby a primary winding 11 and a laminated magnetic core 10.

Referring to Fig.2, in addition to the pri mary winding 11, which islocated within the refractory tube 7, there is provided a second windingor coil 12, which is energized in parallel with the primary 11 andconnected to assist it, that is, so as to produce magnetic flux in thesame direction as the primary winding. This second winding is providedwith taps 14 and a switch mechanismindicated generally at 13 for thepurpose of varying the number of efi'ective turns of this secondwinding.

Each of the figures of the drawing indicates clearly the large area forflux leakage that exists in an electric induction furnace between theprimary winding 11 and the secwearer ondary winding which is comprisedby the single turn 9 of molten metal. As described above, the magneticflux produced by the primary winding 11 is in one direction and thatproduced by the secondary winding 9 is in the opposite direction, andwhen these fluxes attempt to neutralize one another a portion of theprimary flux is diverted from its path in the iron core and sentthroughthe space between the primary and secondary so that it does notintcrlink the primary coil with the secondary coil. The presence of thesecond coil 12, however, which is connected so as to assist the coil 11,causes the majority of the primary flux to stay within the iron core andmagnetically interlink both the primary and secondary.

Referring to Fig. 3, it will be seen that the second coil 12 is providedat one end with a predetermined number of turns of increased crosssection and suitable taps forming the secondary of an auto-transformerof which the whole coil 12 is the primary. Appropriate switching mechanism 19 connects the primary Winding 11 with the secondary of thisauto-transformer and by placing the switch 19 on the various taps 16, 17and 18, the secondary of this auto-transformer can be made to either addto or subtract from the voltage applied to the primary 11. By this meansthe current taken by the furnace can be adjusted within veryconsiderable limits to compensate for the wear of the secondary channelcaused by the molten metal or for materials of different electricalconductivity which it may be desired to melt in the furnace.

This invention not only provides an effective means of current controlbut the use of the secondary primary coil 12 as a means for improvingpower factor is very advantageous as it placesthe user of the electricfurnace in a much better position in contracting for power. It alsopermits the use of a smaller amount of copper in the main primarywinding and reduces the heat loss due to electrical resistance in thiswinding.

I claim:

1. In an electric induction furnace having a loop-shaped channel and amagnetic core linked therewith, a primary winding on said core locatedwithin the loop of said channel, and a second winding located on saidcore outside of said loop, both said windings being adapted to be soconnected to an external source of current as to be thereby energized inparallel.

2. In an electric induction furnace having a loop-shaped channel and amagnetic core linked therewith, a primary winding on said core locatedwithin the loop of said channel, a second winding located on said coreoutside of said loop and means for varying the number of turns in thesecond winding.

3. In an electric induction furnace having a loop-shaped channel and amagnetic core linked therewith, a primary winding on the core, a part ofsaid winding being located within the loop of the channel and a partoutside of said loop, and means for varying the efiective currents ofthat part of the winding located outside of said loop.

4. In an electric induction furnace having a loop-shaped channel and amagnetic core linked therewith, a primary winding on said core embracedby the loop of said channel, a winding on said core located outside saidloop, and means for varying the number of effective turns of the latterwinding,

5. In an electric induction furnace having a loop-shaped channel and amagnetic core linked therewith, a primary winding on the core locatedwithin the loop of the channel, a'second winding located on the coreoutside of said loop, and means for connecting the primary winding inseries with a portion of the second winding.

In testimony whereof I hereunto atlix my signature.

CHARLES B. FOLEY.

